1. Field of the Invention
The present invention relates to a magnetometer and, more particularly, to a magnetometer using a plurality of superconducting quantum interference devices (SQUIDs).
2. Discussion of the Related Art
A general direct current superconducting quantum interference device (dc SQUID), as shown in FIG. 1A, is composed of a superconducting loop c of low inductance having two superconducting junctions a and b. It is electromagnetically characterized in that, when direct bias current Io flows to the two superconducting junctions a and b, a voltage VJ of the superconducting junctions a and b are changed with regard to magnetic flux .PHI.a passing through the loop c of a superconducting quantum interference device (SQUID), as shown in FIG. 1B. This voltage Vj is known by a function of magnetic flux oscillating in a period of one flux quantum, as shown in FIG. 1C.
In a magnetometer using SQUIDs, there is formed a magnetic flux-locked loop having a negative feedback part for keeping the amount of magnetic flux passing through the superconducting loop constant. Magnetometers using SQUIDs are classified into a non-modulating mode and a modulating mode, as shown in FIGS. 2B and 2C.
A magnetometer, whether it is a non-modulating mode or a modulating mode, as shown in FIG. 2A, includes a detecting part 1, a signal-processing part 2, and a feedback part 3.
The detecting part 1 is composed of SQUIDs 4 and 4a for converting external magnetic flux passing through the loop of the SQUIDs 4 and 4a into a voltage VJ at superconducting junctions.
The feedbacking part 3, composed of resistors 15 and 15a and coils 16 and 16a, feedbacks the magnetic flux to the loop of the SQUIDs 4 and 4a when current of as much as the value which is the output voltage Vo divided by resistors 15 and 15a flows to the coils 16 and 16a.
The other components shown in FIGS. 2B and 2C belong to the signal-processing part 2 which modulates, amplifies, and demodulates the voltage converted by the detecting part 1.
If the changed amount of magnetic flux versus voltage (dV/d.PHI.) is bigger than the noise voltage of a pre-amplifier (see FIG. 4 of U.S. Pat. No. 5,122,744), signal-resolving ability of a magnetometer isn't affected by noise voltage of the amplifier 8a even though a non-modulating mode is selected to directly connect a SQUID with the amplifier 8a. Accordingly, the magnetometer is simply composed of a SQUID 4a, an amplifier 8a, a resistor 15a, and a coil 16a as shown in FIG. 2B.
The operations of a magnetometer shown in FIG. 2B will be described in detail. A voltage VJ is generated at both ends of the SQUID 4a if external magnetic flux .PHI.a enters the loop of the SQUID 4a. The voltage VJ is enough amplified by the amplifier 8a to be outputted as a voltage Vo at both ends of the resistor 15a, and current If of as much as the value of the output voltage Vo divided by the resistor 15a flows to the coil 16a. The current If constantly keeps the total amount of magnetic flux passing through the loop of the SQUID 4a by feedbacking magnetic flux to the loop of The SQUID 4a. Accordingly, the value of the output voltage of the magnetometer with regard to the external magnetic flux .PHI.a becomes linear, and the output voltage is read so that the value of the external magnetic flux .PHI.a can be known.
As for a modulating mode, if dV/d.PHI. is smaller (seen the article of Clarke, J., Goubau, W. M., and Ketchen, M. B. J. Low. Tem. Phys. 25, 99-144 (1976)), at the magnetometer of a modulating mode as shown in FIG. 2C, current Im generated by local oscillator 11 and the resistor 12 imposes alternating current magnetic flux to the SQUID 4 through the coil 16 so as to modulate the amount of changed voltage with regard to the external magnetic flux .PHI.a of the SQUID 4. A transformer 6 and a capacitor 7 are inserted between the SQUID 4 and the first amplifier 8 to form a resonator, thus obtaining optimal noise impedance matching between the SQUID 4 and the first amplifier 8 in a resonating frequency. A modulated signal passing through the first amplifier 8 and alternating current signal provided by the local oscillator 11 and the phase shifter 10 are multiplied by a multiplier 9.
An output voltage of the multiplier 9 passes through the second amplifier 13 and the integrator 14 to be demodulated as a voltage Vo at both ends of the resistor 15, and current If of as much as the value which is the output voltage Vo divided by the resistor 15 flows to the coil 16. The current If feedbacks magnetic flux to the loop of the SQUID 4 so as to keep constant the total amount of the magnetic flux passing through the loop of the SQUID 4. As a result, the value of output voltage of the magnetometer becomes linear with regard to the external magnetic flux .PHI.a, and by reading the output voltage, the value of the external magnetic flux .PHI.a can be known.
However, the conventional magnetometer has problems. When a SQUID measures a low frequency signal such as electrocardiogram or electroencephalograph by means of a low frequency noise shown in FIG. 1D or by means of external noise entering by wires or a SQUID, the ability of resolving external magnetic flux signals of the magnetometer becomes inferior.